1. Field of the Invention
The present invention relates to an apparatus for the continuous production of an aromatic carboxylic acid by liquid-phase oxidizing a corresponding alkyl aromatic compound with a molecular oxygen-containing gas in the presence of a lower aliphatic carboxylic acid solvent and an oxidation catalyst. More particularly, the invention relates to an improved reaction vessel which can be used to conduct the oxidation reaction of producing an aromatic carboxylic acid sparingly soluble in a solvent in a stable manner for a long period of time without being accompanied by the adhesion of the aromatic carboxylic acid produced on the inside wall of the reaction vessel at about the interface between the vapor-phase portion and the liquid-phase portion of the reaction system.
2. Description of the Prior Art
In producing an aromatic carboxylic acid such as, for example, terephthalic acid by the liquid-phase oxidation of the corresponding alkyl aromatic compound such as p-xylene, a continuous system wherein the liquid-phase oxidation is carried out continuously while continuously supplying a raw material alkyl aromatic compound, a solvent, an oxidation catalyst, and a molecular oxygen-containing gas into the reaction vessel and, on the other hand, continuously withdrawing the reaction product aromatic carboxylic acid from the bottom of the reaction vessel is more advantageous than a batch system because in the continuous system a product having uniform quality can be obtained in a stable manner over a long period of time.
However, in producing an aromatic carboxylic acid such as terephthalic acid using a lower aliphatic carboxylic acid such as acetic acid as the solvent, the aromatic carboxylic acid gradually crystallizes out if the reaction vessel is used for a long period since the aromatic carboxylic acid which is the reaction product is sparingly soluble in the solvent and the crystals accumulate and adhere to the inside wall, etc., of the reaction vessel. In particular, it has been confirmed that in a reaction vessel equipped with a stirrer used in a conventional reaction, a large amount of crystals adhere to the inside wall of the reaction vessel near the interface between the vapor-phase portion and the liquid-phase portion due to the influence of the stirring. If the crystals of the reaction product adhere to the inside wall of the reaction vessel as mentioned above, the effect of the stirring is decreased and the volume for the liquid portion in the reaction vessel is reduced. This makes it difficult to continue the operation in a stable manner for a long period of time. Also, since in such a case the heat of reaction accumulates in the liquid portion due to the reduction in the area of the reaction liquid surface for evaporation, the occurence of side reactions increases to reduce inevitably the yield for and the quality of the reaction product.
Therefore, in such a conventional operation for the production of an aromatic carboxylic acid, the operation inevitably must be stopped when the adhesion and accumulation of the reaction product on the inside wall of the reaction vessel becomes great, the inside of the reaction vessel must be sufficiently washed with an aqueous solution of an alkali such as sodium hydroxide to remove the accumulated material, and thereafter the operation is resumed. However, thus interruption as described above, as a matter of course, reduces the operation efficiency, results in an uneven reaction product quality throughout the long run, and increases the cost of materials and labor required for the operation.
Also, as disclosed in Japanese patent publication No. 5140/'71, an improved apparatus for producing terephthalic acid is proposed in which a small cylinder closed at the bottom is placed concentrically in a cylindrical reaction vessel to provide inside the reaction chamber a means of preventing the adhesion of terephthalic acid on the inside wall of the apparatus and a raw material mixture heated to a temperature substantially the same as the reaction temperature is supplied to the annular space between the wall of the outer cylindrical reaction vessel and the wall of the inside reaction chamber to allow the mixture to flow over the wall edge of the inside reaction chamber as an overflow or droplets of it. However, in such a proposed apparatus a double-walled complicated apparatus having a quite different structure from that of a conventional apparatus must be used and also it is necessary to pre-heat the raw material mixture to a temperature substantially the same as the reaction temperature. That is, the employment of such an apparatus is operationally and economically disadvantageous.
Thus, a satisfactory reaction vessel which can be used economically and easily for producing an aromatic carboxylic acid efficiently without being accompanied by adhesion of crystals of the aromatic carboxylic acid on the inside wall of the reaction vessel has not yet been developed.